Allow empty m4 arguments

For example:
    ifelse(`$1',,, `
        refpolicywarn(`dollar one defined')
    ')

Author: cgzones

src/parse.y

@@ -706,6 +706,8 @@ m4_args:
 	;
 
 m4_argument:
+	%empty
+	|
 	BACKTICK SINGLE_QUOTE
 	|
 	BACKTICK lines SINGLE_QUOTE

tests/sample_policy_files/uncommon.te

@@ -123,3 +123,7 @@ ifdef(`foo',`
 define(`not_foo',`-foo_t')
 
 define(`foobar')
+
+ifelse(`$1',,, `
+    refpolicywarn(`dollar one defined')
+')